1. Field of the Invention
The present invention relates generally to semiconductor memory devices, and more particularly to a semiconductor memory device capable of switching modes according to the usage environment.
2. Description of the Background Art
FIG. 5 is a partial diagram of a conventional SRAM (Static Random Access Memory) disclosed in Japanese Patent Laying-Open No. 61-190786. FIG. 5 shows the structure of a 4row-4column SRAM as an example. Referring to FIG. 5, an X decoder 1 is responsive to an entry of an X address to provide a signal activating any of NOR gates 2a-2d in a word line actuating circuit 2. The output of NOR gates 2a-2d is provided to word lines 3a-3d, respectively. A Y decoder 4 is responsive to an entry of a Y address to provide a signal activating any of NOR gates 5a-5d in a bit line actuating circuit 5. Four bit line pairs of 6a and 6b-9a and 9b are provided to cross word lines 3a-3d arranged in parallel. Memory cells MC00, MC01, . . . , MC33 are provided at each intersection of the word line and the bit line pair. For example, a high resistance load type memory cell of FIG. 6, a CMOS type memory cell of FIG. 7, and the like are used as one of the above mentioned memory cells. A bit line pair 6a and 6b is connected to a first voltage supply 15 via bit line load transistors 10a and 10b, as well as to an I/O line pair 20a and 20b via transfer gates 16a and 16b. A bit line pair 7a and 7b is connected to first voltage supply 15 via bit line load transistors 11a and 11b, as well as to an I/O bit line pair 20a and 20b via transfer gates 17a and 17b. A bit line pairs 8a and 8b is connected to first voltage supply 15 via bit line load transistors 12a and 12b, as well as to I/O line pair 20a and 20b via transfer gates 18a and 18b. A bit line pair 9a and 9b is connected to first voltage supply 15 via bit line load transistors 13a and 13b, as well as to I/O line pair 20a and 20b via transfer gates 19a and 19b. The outputs of NOR gate 5a, NOR gate 5b, NOR gate 5c, and NOR gate 5d in bit line actuating circuit 5 are applied to the gates of the transfer gates 16a and 16b, 17a and 17b, 18a and 18b, 19a and 19b, respectively.
A local sense amplifier 21 is a current output type differential amplifier for amplifying the potential difference of I/O lines 20a and 20b and providing the same as a current signal. Local sense amplifier 21 is activated on receiving the output of a local sense amplifier selecting circuit 22. Local sense amplifier 21 is implemented with transistors 21a, 21b and 21c. I/O lines 20a and 20b are connected to the bases of transistors 21a, and 21b, respectively. The emitters of transistors 21a and 21b are connected to a second voltage supply 30 via transistor 21c. The gate of transistor 21c is supplied with the output of local sense amplifier selecting circuit 22. The collectors of transistors 21a and 21b are connected to readout data buses 23a and 23b, respectively.
A writing driver 29 is responsive to outputs 32a and 32b of a writing amplifier 31 for bringing one of I/O lines 20a and 20b to a high level, and the other to a low level. Writing driver 29 is implemented with transistors 25-28. Transistor 25 has the drain connected to first voltage supply 15 and the source connected I/O line 20a. Transistor 26 has the drain connected to I/O line 20a and the source connected to second voltage supply 30. Transistor 27 has the drain connected to first voltage supply 15 and the source connected to I/O line 20b. Transistor 28 has the drain connected to I/O line 20b and the source connected to the second voltage supply 30. Outputs 32a and 32b of writing amplifier 31 are applied to the gates of transistors 25 and 28, and transistors 26 and 27, respectively.
A clamp potential generating circuit 33 generates a control potential for transistors 40a and 40b to clamp the potentials of readout data buses 23a and 23b. Clamp potential generating circuit 33 is implemented with a diode 34, a resistor 36, and a transistor 37. Diode 34 has its anode connected to first voltage supply 15. The base of transistor 37 is applied with a reference potential for generating constant current via a terminal 35. Transistor 37 has the collector connected to the cathode of diode 34, and the emitter connected to second voltage supply 30 via resistor 36.
A main sense amplifier 38 is a voltage output type amplifier for amplifying current signals passing readout data buses 23a and 23b to invert the same into voltage signals. Main sense amplifier 38 comprises resistors 39a and 39b having one end connected to first voltage supply 15; and transistors 40a and 40b having the base supplied with the output of clamp potential generating circuit 33, the emitter connected to readout data buses 23a and 23b, and the collector connected to the other ends of resistors 39a and 39b. Transistors 40a and 40b are used as clamp transistors for readout data buses 23a and 23b. Main sense amplifier 38 further comprises emitter-follower-transistors 41a and 41b having the bases supplied with the outputs of resistors 39a and 39b, diodes 42a and 42b for level shifting having the anodes connected to the emitters of transistors 41a and 41b, and current source circuits 43-46 connected to establish a constant current source with a transistor and a resistor.
The output of main sense amplifier 38 is applied to an output circuit 47 operating at an ECL (Emitter Coupled Logic) level.
The operation of the conventional semiconductor memory device of FIG. 5 is described hereinafter.
When memory cell MC00 is to be selected for example, signals of a low level are applied to the two inputs of NOR gate 2a in word line actuating circuit 2 from X decoder 1. In response, the output of NOR gate 2a attains a high level to bring word line 3a to a high level. At least one of the two inputs of other NOR gates 2b-2d in word line actuating circuit 2 is supplied with a signal of a high level. This brings the other word lines 3b-3d to a low level. By this operation, word lines 3a is selected. The selection of a bit line is carried out in a similar manner. That is to say, signals of a low level are provided to the two inputs of NOR gate 5a in bit line actuating circuit 5, whereby the output of NOR gate 5a attains a high level. In response, transfer gates 16a and 16b are conductive to select bit line pair 6a and 6b.
FIG. 8 is a diagram of memory cell MC00 of FIG. 5 and the relating reading/writing system. The operation of reading from or writing to the selected memory cell MC00 a data signal is described hereinafter with reference to FIG. 8.
It is assumed that the internal node N1 of memory cell MC00 is at a high level, and node N2 is at a low level. Transistor Q1 in the memory cell is non-conductive, and transistor Q2 is conductive at this time.
When reading, outputs 32a and 32b of writing amplifier 31 are both fixed to the low level. When word line 3a is selected at a high level, transfer gates Q3 and Q4 of memory cell MC00 are both conductive. For example, if the potential of first voltage supply 15 is GND (=0 V) and the potential of second voltage supply 30 is VEE, potential VB1 of bit line 6a is VB1=-VLS. VLS indicates the drop of voltage when current does not flow through bit line load transistor 10a. Potential VB2 of bit line 6b falls down an extra .DELTA..sup.V in voltage due to ON resistance of bit line load transistor 10b, to result in VB2=-.DELTA..sup.V -VLS. .DELTA..sup.V is called the bit line amplitude, and is usually 50 mV-500 mV. .DELTA..sup.V is adjusted according to the magnitudes of the bit line load transistors 10a and 10b. This bit line amplitude appears on I/O lines 20a and 20b via transfer gates 16a and 16b.
The above bit line amplitude is amplified by local sense amplifier 21 and provided to readout data buses 23a and 23b as a current signal. More specifically, bit line 6a is at a high level and bit line 6b is at a low level, to bring I/O line 20a to a high level and I/O line 20b to a low level. This causes transistor 21a connected to I/O line 20a to become conductive, and transistor 21b connected to I/O line 20b to become non-conductive in local sense amplifier 21. Therefore, when transistor 21c is turned on by the output of sense amplifier selecting circuit 22, sensing current flows through readout data line 23a, but not through readout data line 23b.
The potentials of readout data buses 23a and 23b are clamped to a constant potential VCL by clamp potential generating circuit 33 and readout data bus clamp transistors 40a and 40b. Clamp potential VCL is determined by output potential -VD of clamp potential generating circuit 33 and the voltage VBE between the base and emitter of readout data bus clamp transistors 40a and 40b. More specifically, VCL=-VD-VBE.
The readout rate is increased in speed due to the potentials of readout data buses 23a and 23b clamped to a constant potential VCL by readout data bus clamp transistors 40a and 40b. The reason for this benefit is described hereinafter with reference to an equivalent circuit of FIG. 9.
Because readout data buses 23a and 23b are wired along the long or short side of the semiconductor chip, the wiring capacitance thereof is great. In addition, many local sense amplifiers 21 are connected to the same readout data bus to result in a great value in collector capacitance. If readout data bus clamp transistors 40a and 40b are not utilized, resistors 39a and 39b are directly connected to readout data buses 23a and 23b. Voltage drop according to the current across resistors 39a and 39b influence the potentials of readout data buses 23a and 23b, whereby the potentials of readout data buses 23a and 23b will vary according to the magnitude of the current signal provided from local sense amplifier 21. For example, if the potential difference between readout data buses 23a and 23b corresponding to a readout data of logic "0" and a readout data of logic "1" is 0.5 V, the potentials of readout data buses 23a and 23b must be switched at a signal amplitude of 0.5 V in accordance with the readout data. As mentioned in the foregoing, readout data buses 23a and 23b have a great load capacitance C. This means that a period of time is required for charging/discharging of load capacitance C, not allowing rapid change in potentials of readout data buses 23a and 23b. As a result, change in output voltage is slow to lower the readout rate. On the other hand, if readout data bus clamp transistors 40a and 40b are used, potentials of readout data buses 23a and 23b are kept at a constant potential VCL, whereby charge/discharge of load capacitance C is eliminated to change the output voltage at high speed. This improves the readout rate significantly.
Referring to FIG. 8 again, sensing current crosses resistor 39a via readout data bus clamp transistor 40a in main sense amplifier 38. This causes the output of resistor 39a to be greater than that of resistor 39b in voltage drop by the sensing current component to provide a low level signal from transistor 40a.
The potential difference between resistors 39a and 39b is provided to an output buffer 47 via emitter-follower-transistors 41a and 41b and level shifting diodes 42a and 42b to end the reading operation of memory cell MC00.
When writing, the potential of one bit line of the pair is brought to a high level, and the potential of the other bit line is brought to a low level. For example, in order to write an invert data (data of logic "0") into memory cell MC00, output 32a of writing amplifier 31 is brought to a low level and 32b to a high level, whereby transistors 25 and 28 of writing driver 29 is rendered non-conductive, and transistors 26 and 27 rendered conductive. This causes I/O line 20a to attain a low level and I/O line 20b a high level to bring bit lines 6a and 6b to a low level and a high level, respectively. As a result, an invert data (data of logic "0") is written into memory cell MC00.
It is necessary for manufacturers to prepare various kinds of products in providing the above mentioned semiconductor memory devices to users. This necessity is due to the following reasons. The first reason is that variation in bit organizations of input/output data calls for a variety of types. If the bit organization of a system utilizing a semiconductor memory device is .times.N (N is a positive integer of 1 and above), the semiconductor memory device must be able to input and output data by N bits. If the bit organization of the input/output data changes, it will become necessary to modify the number of pins and the internal circuit in a semiconductor memory device for data input/output. This results in a necessity of providing variety of types to suit each bit organization. The second reason is that the variety of packages where semiconductor chips are mounted calls for the provision of various types of semiconductor memory devices. FIGS. 10 and 11 show two typical types of packages. Package PA1 of FIG. 10 is called a DIP, in which input/output pins are arranged along the long side. A semiconductor chip SUC1 mounted on the package must have bonding pads arranged along the long sides to fit the package. Package PA2 of FIG. 11 is called a flat package having input/output pins arranged along the short sides. A semiconductor chip SUC2 mounted on that package must have bonding pads arranged along the short side to fit that package. Hence, various types of semiconductor memory devices must be prepared according to the types of packages to be mounted. The third reason is that the various operation modes (for example, nible mode, page mode in dynamic RAMs) employed by the system calls for various types of semiconductor memory devices. The locations of input/output data pins in a semiconductor memory device must be changed if the operation mode employed by the system is changed. It is therefore necessary to provide various types of semiconductor memory devices according to the employed operation mode.
The need to produce various types of semiconductor memory device products is a great load for manufacturers. For example, various types of semiconductor memory devices must be designed, and a production line must be provided for each type of semiconductor memory device. This will prevent reduction in the product cost of mass production, resulting in expensive products for the user. It is desirable to develop and manufacture a variety of products with minimum labor for manufacturers.
To satisfy the above requirements, some recent semiconductors are provided with several types of bonding pads arranged on the semiconductor chip for identical integration density, whereby the connection between the internal circuitry of the semiconductor integrated circuit device and the bonding pads are switched according to the usage environment. For example, FIG. 12 shows a semiconductor chip SUC3 provided with bonding pads BP1 suitable for a DIP and bonding pads BP2 suitable for a flat package. If semiconductor chip SUC3 is mounted on a DIP, the internal circuitry and bonding pads BP1 are connected. If semiconductor chip SUC3 is mounted on a flat package, the internal circuitry and bonding pads BP2 are connected. This allows the usage of the same semiconductor memory device in a plurality of environments to reduce load of manufacturers caused by increase in variety.
The methods of switching the connection between the internal circuitry of a semiconductor memory device and bonding pads take conventional methods such as those shown in FIGS. 13 and 14.
The method of FIG. 13 switches the connection between various types of bonding pads BP1-BPn and the internal circuitry at the time of the wiring step. This method is called master slicing.
The method of FIG. 14 inserts switches such as transfer gates TG1-TGn between a plurality of types of bonding pads PB1-PBn and the internal circuitry to switch connection between the bonding pads and the internal circuitry by the on/off thereof. This method is called bonding option, where external switching control signals of transfer gates TG1-TGn are applied to different bonding pads.
The above-mentioned switching method by master slicing or using transfer gates will be explained hereinafter in details applied to a SRAM of a Bi-CMOS.
According to increase in integration density, recent semiconductor memory devices usually have a structure where a memory cell array is divided into a plurality of blocks using divided word line techniques such as that disclosed in Japanese Patent Publication No. 62-28516. If a memory cell array is divided into a plurality of blocks, the number of bit line pairs connected to the same I/O line pair is reduced to increase the speed of access time. For the purpose of assisting design modification of semiconductor memory devices of different bit organizations, a plurality of local sense amplifiers (4-16, for example) are arranged in the same block to multiplex these outputs, allowing data input/output of a bit organization different from that in the semiconductor memory device.
FIGS. 15 and 16 are block diagrams showing the structures of semiconductor memory devices including the aforementioned block division and local sense amplifier division. In FIGS. 15 and 16, a memory cell array is divided into two blocks, each having two local sense amplifiers. FIG. 15 is an example of a x1 organization semiconductor memory device. FIG. 16 is an example of a x2 organization semiconductor memory device.
Referring to FIG. 15, a memory cell array is divided into a first block L and a second block R by word line division. The first block L and the second block R each comprise two sub-blocks. That is to say, the first block L comprises sub-blocks 70.sub.L1 and 70.sub.L2. The second block R comprises sub-blocks 70.sub.R1 and 70.sub.R2. Associated with sub-blocks 70.sub.L1, 70.sub.L2, 70.sub.R1 and 70.sub.R2, bit line actuating circuits 5.sub.L1, 5.sub.L2, 5.sub.R1, and 5.sub.R2, writing amplifiers 31.sub.L1, 31.sub.L2, 31.sub.R1, and 31.sub.R2, and local sense amplifiers 21.sub.L1, 21.sub.L2, 21.sub.R1, and 21.sub.R2 are provided. X address decoder 1 decodes an externally applied X address signal and provides the decoded signal to word line actuating circuit 2. In response, word line actuating circuit 2 actuates one word line of either the first block L or the second block R. Y address decoder decodes an externally applied Y address signals and applies the decoded signal to bit line actuating circuits 5.sub.L1, 5.sub.L2, 5.sub.R1, and 5.sub.R2. Each bit line actuating circuit opens the transfer gate of the selected bit line pair (refer to FIG. 5) according to the decoded signal from Y address decoder 4. Reading/writing control circuit 73 selectively actuates writing amplifiers 31.sub.L1, 31.sub.L2, 31.sub.R1 and 31.sub.R2, and local sense amplifiers 21.sub.L1, 21.sub.L2, 21.sub.R1 and 21.sub.R2. Decode circuit 78 decodes a selecting signal D for providing a control signal to comply the writing and reading system of the semiconductor memory device with external x1/internal x2 organization. A signal of the more significant several bits of the Y address applied to Y address decoder 4, for example, is used as selecting signal D. In case of internal x2 organization, the most significant bit (MSB) of the Y address is used. Decode circuit 78 provides selecting signal D and the inverted signal D! as control signals. ! indicates inversion in the present specification and drawings. Control signals D, D! are applied to a demultiplexer 79 and a multiplexer 81. Demultiplexer 79 provides the x1 data input to either of writing data buses 76.sub.1 or 76.sub.2 according to control signals D, D! from decode circuit 78. Writing data bus 76.sub.1 is connected to writing amplifiers 31.sub.L1 and 31.sub.R1. Writing data bus 76.sub.2 is connected to writing amplifiers 31.sub.L2 and 31.sub.R2. The outputs of local sense amplifiers 21.sub.L1 and 21.sub.R1 are applied to main sense amplifier 38.sub.1 via readout data bus 77.sub.1. The outputs of local sense amplifiers 21.sub.L2 and 21.sub.R2 are provided to main sense amplifier 38.sub.2 via readout data bus 77.sub.2. Multiplexer 81 is responsive to control signals D, D! from decode circuit 78 to multiplex and provide the outputs of main sense amplifiers 38.sub.1 and 38.sub.2 to output buffer 47. Thus, x1 data is provided from output buffer 47.
In the semiconductor memory device of FIG. 15, demultiplexer 79 provides the input data to either of data buses 76.sub.1 or 76.sub.2 according to control signals D, D! from decode circuit 78, when x1 data is applied to demultiplexer 79 at the time of writing. At this time, one memory cell in either of the sub-blocks is selected by X address decoder 1 and Y address decoder 4. Then, a writing sense amplifier corresponding to the sub-block of the selected memory cell is selectively enabled by reading/writing control circuit 73. x1 input data is written into the selected memory cell via the selectively enabled writing amplifier.
At the time of reading, a memory cell in either of the sub-blocks is selected by X address decoder 1 and Y address decoder 4. A local sense amplifier corresponding to the sub-block of the selected memory cell is selectively enabled by reading/writing control circuit 73. Data readout from the selected memory cell is sensed in the corresponding local sense amplifier, and applied to either of main sense amplifiers 38.sub.1 or 38.sub.2 via either of readout data buses 77.sub.1 or 77.sub.2. At this time, multiplexer 81 is responsive to control signals D, D! from decode circuit 78 and switched to select and provide the output of either of main sense amplifiers 38.sub.1 or 38.sub.2 corresponding to the selected memory cell.
FIG. 16 shows a x2 organization semiconductor memory device in comparison with the x1 organization semiconductor memory device of FIG. 15. The structure of the semiconductor memory device of FIG. 16 is similar to that of FIG. 15 except for the points explained in the following. Corresponding elements have identical reference characters denoted, and the description thereof will not be repeated.
Referring to FIG. 16, input data of parallel 2 bits are directly applied to writing data buses 76.sub.1 and 76.sub.2 via input buffers 83.sub.1 and 83.sub.2, respectively. Data readout on readout data bus 77.sub.1 is amplified by a main sense amplifier 38.sub.1 ' and provided via an output buffer 47.sub.1 '. Similarly, data readout on readout data bus 77.sub.2 is amplified by a main sense amplifier 38.sub.2 ' and provided via an output buffer 47.sub.2 '.
In the semiconductor memory device of FIG. 16, either the first block L or the second block R of the memory cell array is selected at the time of data writing/reading. Corresponding two memory cells are selected simultaneously from the two sub-blocks of the selected block. The two bits of data, i.e. x2 input data supplied simultaneously from data input buffers 83.sub.1 and 83.sub.2 are written simultaneously into the two selected memory cells. At the time of reading, data readout from the corresponding two memory cells selected simultaneously are applied to main sense amplifiers 38.sub.1 ' and 38.sub.2 ' at the same time via readout data buses 77.sub.1 and 77.sub.2, respectively, and amplified.
The selection of x1 organization of FIG. 15 or the x2 organization of FIG. 16 for a semiconductor memory device is conventionally implemented with a mask option. All the functional blocks that can comply with both the x1 organization and the x2 organization are provided on one chip. By switching masks in wiring steps, for example, either the x1 organization or the x2 organization can be selected. That is to say, change in bit organization is performed by the aforementioned master slice method.
Change in bit organization realized by the mask option, i.e. the master slicing method poses various problems. For example, the master slicing method necessitates a plurality of masks in one step for production, due to switching carried out at the wiring step, to increase design and manufacturing costs. Another problem is that the semiconductor memory device cannot be used for a different bit organization after being manufactured.
Semiconductor memory devices switching the bit organization with the aforementioned bonding option has an advantage that it can be commonly used for several usage environments even after being manufactured. Such a semiconductor memory device is disclosed in U.S. Pat. No. 4,907,203, for example. This patent implements switching between x1 organization and x4 organization by switching the connections of the readout data buses and bonding pads with a signal switching circuit in a SRAM of a CMOS.
A structure of a semiconductor memory device is described hereinafter inferred from the technology of U.S. Pat. No. 4,907,203 applied to a SRAM of a Bi-CMOS of FIGS. 15 and 16.
FIG. 17 is a block diagram showing a structure of a SRAM of a Bi-CMOS. The semiconductor memory device of FIG. 17 is implemented to allow the control of switching between x1 organization and x2 organization from an external source. The major structure thereof is a combination of the structures of the semiconductor memory devices of FIGS. 15 and 16. Elements corresponding to those in the semiconductor memory devices of FIGS. 15 and 16 have identical reference characters denoted, and the description thereof will not be repeated.
Referring to FIG. 17, an external switching control signal for controlling the switching between x1 organization and x2 organization is applied to a buffer 86. The output of buffer 86 is applied to a data input selecting circuit 87. Data input selecting circuit 87 is responsive to the switching control signal from buffer 86 to selectively switch between the x1 data input from demultiplexer 79 and the x2 data input from input buffers 83.sub.1 and 83.sub.2, and provides the selected output to writing data buses 76.sub.1 and 76.sub.2.
Readout data buses 77.sub.1 and 77.sub.2 are connected to main sense amplifiers 131.sub.1 and 138.sub.2 for x1, and main sense amplifiers 138.sub.1 ' and 138.sub.2 ' for x2, respectively. The input stages of main sense amplifiers 138.sub.1, 138.sub.2, 138.sub.1 ', and 138.sub.2 ' are provided with current switching means 52.sub.1, 52.sub.2, 52.sub.1 ', and 52.sub.2 ', respectively. Current switching means 52.sub.1 ' and 52.sub.2 ' are supplied with a switching control signal from buffer 86 via a signal line 89. Current switching means 52.sub.1 and 52.sub.2 are supplied with a switching control signal via a signal line 90 which is inverted by inverter 88. Current switching means 52.sub.1, 52.sub.2, 52.sub.1 ', and 52.sub.2 ' serve to control the opening/closing of the current path between readout data buses 77.sub.1 and 77.sub.2 and each sense amplifier, in which the ON/OFF is controlled by a switching control signal and the inverted signal thereof supplied via signal lines 89 and 90. Main sense amplifiers 138.sub.1 and 138.sub.2 for x1 also functions to multiplex data of the two readout data buses 77.sub.1 and 77.sub.2. This multiplex function is controlled by control signals D and D! provided from decode circuit 78. Control signal D is provided to main sense amplifier 138.sub.1 and control signal D! is provided to main sense amplifier 138.sub.2. Main sense amplifiers 138.sub.1 and 138.sub.2 use a wired OR operation for the outputs, whereby the output is provided via output buffer 47.
FIG. 18 is a circuit diagram showing in detail the main feature of the semiconductor memory device of FIG. 17, i.e. the structure of main sense amplifier periphery. The main sense amplifier associated with readout data bus 77.sub.1 and the periphery thereof are shown.
Referring to FIG. 18, local sense amplifier 21.sub.L1 (or 21.sub.R1) is connected to readout data bus 77.sub.1. The structure of local sense amplifier 21.sub.L1 (or 21.sub.R1) is similar to that of local sense amplifier 21 of FIG. 5. Clamp potential generating circuit 133 is implemented with diode 34 having the anode connected to the first voltage supply 15; and a NMOS transistor 37 having the gate supplied with a reference potential for constant current generation via terminal 35, the drain connected to the cathode of diode 34 and a source connected to the second voltage supply 30. Transistor 37 implements a constant current source. Main sense amplifier 138.sub.1 for x1 is of the current detection type, connected to the two readout data lines 23a and 23b of readout data bus 77.sub.1 via PMOS transistors 52a and 52b in current switching means 52.sub.1.
More specifically, main sense amplifier 138.sub.1 is implemented with PMOS transistors 39a and 39b, bipolar transistors 40a, 40b, 41a, 41b, and NMOS transistors 43 and 44. PMOS transistors 39a and 39b are used as variable resistors, having the source connected to first voltage supply 15 and the gate supplied with control signal D from decode circuit 78. Transistor 40a and 40b are transistors for clamping readout data bus 77.sub.1, having the bases supplied with the outputs of clamp potential generating circuit 133, the emitters connected to readout data lines 23a and 23b via PMOS transistors 52a and 52b in current switching means 52.sub.1, and the collectors connected to the drains of PMOS transistors 39a and 39b. Transistors 41a and 41b are used as emitter-follower-transistors, having each collector connected to first voltage supply 15, each emitter connected to the input end of output data buffer 47, and each base connected to the drains of PMOS transistors 39a and 39b, respectively. NMOS transistors 43 and 44 implement a current source, having its gate supplied with a reference potential for constant current source via terminal 35. The other main sense amplifier 138.sub.2 for x1 comprises a similar structure to that of the above mentioned main sense amplifier 138.sub.1. Main sense amplifier 138.sub.2 is connected to readout data bus 77.sub.2 via current switching means 52.sub.2 similar to current switching means 52.sub.1. Main sense amplifier 138.sub.2 is supplied with control signal D!. The outputs of main sense amplifiers 138.sub.1 and 138.sub.2 are connected to implement a wired OR to apply the output to data output buffer 47. NMOS transistors 45 and 46 implement a current source as the common load of main sense amplifiers 138.sub.1 and 138.sub.2. Each gates of transistors 45 and 46 is applied with a reference potential for constant current source via terminal 35.
The structures of a main sense amplifier for x2 and the peripheral circuit thereof are similar to those of the aforementioned main sense amplifier for x1 and the peripheral circuit thereof. Corresponding elements have a prime suffixed to the identical reference character. In the main sense amplifier for x2, main sense amplifier 138.sub.1 ' and main sense amplifier 138.sub.2 ' (refer to FIG. 17) are provided in parallel with one clamp potential generating circuit 133', where each output thereof is applied to output data buffers 47.sub.1 ' and 47.sub.2 ' (refer to FIG. 17), individually. Each gate of transistors 39a' and 39b' in main sense amplifier 138.sub.1 ' is connected to second voltage supply 30 (also similar in main sense amplifier 138.sub.2 '). This causes transistors 39a' and 39b' to be always conductive. Similar to the main sense amplifier of x1, main sense amplifier 138.sub.1 ' for x2 is connected to the two readout data lines 23a and 23b of readout data line 77.sub.1 via PMOS transistors 52a' and 52b' of current switching means 52.sub.1 '. Similarly, main sense amplifier 138.sub.2 ' for x2 is connected to the two readout data lines of readout data line 77.sub.2 via current switching means 52.sub.2 ' (refer to FIG. 17).
The operation of the semiconductor memory device of FIG. 17 and 18 is described hereinafter with particular reference to the characterizing readout operation of the semiconductor memory device.
It is assumed that the semiconductor memory device of FIGS. 17 and 18 is set to a readout state with I/O line 20a at a high level and 20b at a low level. Transistor 21a connected to I/O line 20a is conductive and transistor 21b connected to I/O line 20b is non-conductive at this time. Accordingly, sensing current flows through read out data line 23a, but not through readout data line 23b.
When a main sense amplifier for x1 is to be selected, an external switching control signal of a high level is provided to buffer 86. Current switching means 52.sub.1 and 52.sub.2 are supplied with a switching control signal of a low level via a signal line 90. Current switching means 52.sub.1, and 52.sub.2 ' are supplied with a switching control signal of a high level via a signal line 89. In response, transistors 52a and 52b in current switching means 52.sub.1 and 52.sub.2 become conductive. On the contrary, transistors 52a' and 52b' in current switching means 52.sub.1 ' and 52.sub.2 ' become non-conductive. This causes sensing current flowing through readout data buses 77.sub.1 and 77.sub.2 to flow only through main sense amplifiers 138.sub.1 and 138.sub.2 for x1.
In the main sense amplifier for x1, sensing current flows to PMOS transistor 39a via transistor 40a for clamping readout data buses. Therefore, the output of PMOS transistor 39a is greater than that of PMOS transistor 39b in voltage drop by the sensing current to provide a low level signal.
One of control signals D and D! is brought to a low level indicating selected state and the other brought to a high level indicating a non-selected state. The main sense amplifier supplied with the control signal of a high level indicating a non-selected state has both PMOS transistors 39a and 39b turned off, whereby the output potential thereof are dropped by sensing current or currents of current sources 43 and 44. The corresponding sense amplifier provides a signal of low level. For example, if control signal D! is a high level of non-selection, the corresponding main sense amplifier 138.sub.2 provides a low level signal.
The passage of sensing current through PMOS transistor 39a in the selected main sense amplifier 138.sub.1 generates a voltage drop determined by the ON resistance of PMOS transistor 39a used as a resistance load and the magnitude of sensing current and current of current source 43 to provide a low level signal from PMOS transistor 39a. PMOS transistor 39b which is a companion to PMOS transistor 39a has only the current drawn from current source 44 to provide a signal of high level.
The potential difference between PMOS transistors 39a and 39b is provided via emitter-follower transistors 41a and 41b. This output and the output of main sense amplifier 138.sub.2 go through wired OR operation to be provided to output buffer 47. This achieves the readout operation.
Main sense amplifiers 138.sub.1 ' and 138.sub.2 ' for x2 not used have terminal 35' supplied with voltage of a low level. This turns off transistors 37', 43', 44', 45', and 46' which serve as current sources. This can reduce the current consumption.
When main sense amplifiers 138.sub.1 ' and 138.sub.2 ' for x2 are to be selected, an external switching control signal of a low level is applied to buffer 86. Main sense amplifiers 138.sub.1 and 138.sub.2 for x1 are supplied with a switching control signal of a high level via signal line 90. Main sense amplifiers 138.sub.1 ' and 138.sub.2 ' for x2 are supplied with a switching control signal of a low level via signal line 89. This causes transistors 52a and 52b of current switching means 52.sub.1 and 52.sub.2 to become non-conductive, and transistors 52a' and 52b' in current switching means 52.sub.1 ' and 52.sub.2 ' to become conductive. Sensing current flowing through readout data lines 23a and 23b of readout data buses 77.sub.1 and 77.sub.2 flows through only main sense amplifiers 138.sub.1 ' and 138.sub.2 ' for x2. Because each output of main sense amplifiers 138.sub.1 ' and 138.sub.2 ' for x2 is separately provided to output buffers 47.sub.1 ' and 47.sub.2 ', respectively, main sense amplifiers 138.sub.1 ' and 138.sub.2 ' are always at a selected state. That is to say, the semiconductor memory device is implemented to provide simultanouesly data of two bits from two sense amplifiers 138.sub.1 ' and 138.sub.2 '. The other readout operation is similar to the case where main sense amplifiers 138.sub.1 and 138.sub.2 for x1 are selected.
The semiconductor memory device of FIGS. 17 and 18 is superior in adaptability in comparison with the aforementioned semiconductor memory device performing switching by master slicing, because the former can have the organization switched in accordance with the usage environment even after being manufactured. However, the semiconductor memory device of FIGS. 17 and 18 has a problem that the current switching means provided between the readout data bus and each main sense amplifier prevents increase in speed of readout rate by the readout data bus clamp transistor. This is because the voltage drop of the transistors (for example, PMOS transistors 52a, 52b) within the current switching means changes according to the current flowing through readout data buses to affect the potential of the readout data bus. If the potential of the readout data bus changes, charge/discharge with respect to load capacitance of the readout data bus occurs to reduce the readout rate.